Basically, mounts for optical elements are constructed based on requirements for imaging quality and the given conditions of transport, storage and use of the optical system of which the mounted optical element forms a component part. Particularly at issue are anticipated shock loads, possible temperature fluctuations during transport, storage and use, and influences of radiation energy and radiation spectrum during use. These stresses notwithstanding, the optical element must be held in the mount in a defined position durably and under low tension.
In view of the aforementioned requirements for an assembly according to the invention, prior-art assemblies, or mounts as component parts thereof, will be considered as such in the following only insofar as they—like an assembly according to the invention—hold an optical element in a mount which allows a radial expansion compensation between the material of the mount and the material of the optical element over a predetermined temperature range. Assemblies of this type are referred to as thermally compensated.
Mounts of the type mentioned above often comprise a rigid mounting ring and a plurality of elastic links by which an optical element is connected to the mounting ring directly or indirectly via an auxiliary mount. The elastic links compensate for the radially variable expansion of the optical element and mounting ring by deforming in a reversible manner, whereby a reaction force is brought about which counteracts the deformation and acts on the optical element at connection points between the elastic links and the optical element. Through a targeted selection of material and constructional implementation of the links such that they are radially compliant or through special steps for configuring the optical element which are intended to prevent operative forces from leading to stresses in the optically active regions, it has been attempted in the prior art to minimize the effect of differential expansion or to shift the location in which the differential expansions operate.
Laid Open Application DE 10 2006 060 088 A1 discloses an optical assembly having a mounting ring (referred to in the cited reference as “holder”) at which are formed along the inner circumferential surface three elastic links which are integrally connected to the mounting ring and by which the mounting ring is connected to a lens. In each instance the links comprise a flexural element (referred to in the cited reference as “web”) which medially and tangentially contacts the optical element at connection points, and the ends of this flexural element transition into the mounting ring. Because of the radial elastic compliance of the tangentially contacting flexural elements, different thermal expansions between the optical element and the mount can be compensated and the optical element is held under low tension within a given temperature range. The optical element is held so as to be constantly centered. In the basic state of the optical assembly at normal temperature, the flexural elements are relaxed. When there is a change in temperature, they are increasingly tensioned radially so that an increasingly larger reaction force, which presents as a compressive force or a tensile force depending on the direction of the reaction force, acts on the connection points in radial direction.
An optical arrangement known from DE 10 2010 008 756 A1 also has a monolithic mount and an optical element held therein in a rotationally symmetrical manner via three elastic links (referred to in the cited reference as “spring leg arrangements”). The spring leg arrangements are formed in each instance by two parallel spring legs (flexure elements), one end of each of the parallel spring legs transitions into a mounting ring (referred to in the cited reference as “outer mount region”) and the other ends terminate in a contact foot to which the optical element is fixed by gluing or soldering. The two parallel spring legs also act as flexure elements in this case and are arranged so as to be spaced apart from one another in direction of their compliance, i.e., perpendicular to the optical axis of the optical element, the gap therebetween being small in proportion to their length. From the point of view of the optical element, they extend along a concave line of curvature. During a radial expansion of the optical element, this optical element exerts radially operative forces on the contact feet, which results in the deflection of the parallel spring legs in a plane perpendicular to the optical axis. In contrast to a simple spring leg which is fixed on one side, there is no bending moment in the region of contact with the optical element, which would be explained by the fact that the spring leg arrangement itself generates a moment in the contact region which counteracts the torque exerted by the optical element, as a result of which the contact foot can only execute a translational movement. Therefore, the parallel spring legs are increasingly tensioned as the temperature increases over normal temperature, so that an increasingly greater reaction force acts on the connection points in radial direction, presenting a compressive force or a tensile force depending on the direction of the reaction force. An optical assembly with a similar mount is known from DE 10 2010 022 934 A1.
The not-prior-published DE 10 2013 109 185 B3 discloses an optical assembly comprising a rotationally symmetrical optical element and a monolithic mount having a mounting ring and at least three links (referred to in the cited reference as “connection units”) by which the optical element is connected to the mounting ring. The links comprise three interconnected couplers which have certain length ratios with respect to one another and are connected to one another and to the mounting ring via flexure bearings. The couplers are considered to be stiff within the movement range of the link so that the deflection of the link can be represented by a transmission diagram for a coupling transmission. The optical element is fastened to one of the links in each instance via a fastening point which is guided in each instance on a straight line. The flexure bearings are elastically deformed with the deflection of the links so as to cause restoring forces in the flexure bearings which cooperate through flow of force to bring about a reaction force in direction of the straight lines for connection to the associated fastening point.
It has been pointed out in the above-cited references that it is merely advantageous to construct the mounts as monolithic components and that mounts based on the demonstrated principles can also be implemented through discrete components.
A mount assembly with a monolithic mount comprising a mounting ring and elastic connection mechanisms is disclosed in US 2011/0096314 A1. The connection mechanisms are formed in each instance by a chain of connection elements which are connected to one another by flexure bearings. During deflection of the flexure bearings due to differential expansions, a restoring force is also brought about here depending on the degree of deflection and depending on the temperature.
In a mount assembly shown in U.S. Pat. No. 7,139,137 B2, an optical element is held inside a mounting ring indirectly via discrete spring elements. Compression elements mounted in the mounting ring act at the spring elements, and the optical element can be aligned inside the mounting ring through displacement of the compression elements.
US 2010/0097697 A1 discloses a further mount assembly in one embodiment of which referring to FIG. 16 an optical element is held inside a mounting ring via spring arrangements. The spring arrangements are formed in each instance by a cup mounted on the mounting ring in which a rod and a spring arranged in a line act on the optical element. This type of bearing is intended to guard the optical element against damage during transport in particular.
The solutions of the above-cited references have in common that an optical element mounted in a mount is connected to a mounting ring via elastic links so as to compensate temperature-dependent differences in expansion. However, contingent upon the different expansion of the mount and optical element, reaction forces are generated at the connection points and act on the optical element. These reaction forces can lead to stresses or to changes in tension in the optical element and, therefore, change the imaging characteristics of the optical element.
No optical assembly of the type mentioned above is shown in the prior art in which a counterforce counteracting the reaction forces is generated by additional means at the connection points.